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AST 101 Introduction to Astronomy: Stars & Galaxies

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AST 101 Introduction to Astronomy: Stars & Galaxies QUIZ #3 - DISCUSSION Ginger and Fred are two stars. Ginger’s parallax is ½ of Fred’s. Ginger’s apparent brightness is 5 Magnitudes, Fred’s is 10 Magnitudes. a) If Ginger is 1ly year away from us, how far is Fred? [3pt] Distances scale inversely with parallax angle. Since Fred’s parallax is twice that of Ginger, his distance is half that of Ginger, i.e. 0.5ly. AST 101 b) Which of the two has a higher apparent brightness, and by how much? [3pt] Introduction to Astronomy: Ginger is brighter, by a factor of 100, since the difference in magnitude is 5=2*2.5 and each factor of 2.5 in magnitudes yields a power of 10 in flux. Stars & Galaxies c) Which one of the two has a higher intrinsic luminosity, and by how much? [4 pt] Lum = Apparent brightness * (4*pi* D^2) [Inverse square law!] Lum(Ginger)/Lum(Fred) = 100*1^2/(1.*0.5^2)=100/0.25 = 400. Ginger's luminosity is a factor of 400 larger than Fred's. • Top end of main What about the other objects on sequence starts to the H-R diagram? “peel off” As stars run out of Luminosity Luminosity hydrogen fuel their • Pleiades star cluster properties change (generally they turn shown ! no more O into red giants- and B stars more on why later ) Temperature Clicker Question Main- sequence turnoff point How do we measure the age of a stellar of a cluster cluster? tells us its age A. Use binary stars to measure the age of stars in the cluster. B. Use the spectral types of the most numerous stars in the cluster to infer their temperatures, and thus, the age of the cluster. C. Find stars in the instability strip and use their variability period to measure their age. D. Look for the age of stars at the main-sequence turnoff point. E. Determine if the cluster is an open cluster or globular cluster and use the average age of those types of clusters. Clicker Question Clicker Question Main sequence A-stars have masses about 3 How do we measure the age of a stellar times that of the Sun, and luminosities about 30 cluster? times that of the Sun. What is the age of a cluster A. Use binary stars to measure the age of stars in the which has a “turnoff” at A-stars? (Remember: cluster. The Sun’s lifetime ~ 10 billion years) B. Use the spectral types of the most numerous stars in the cluster to infer their temperatures, and thus, the A) 100 thousand years age of the cluster. C. Find stars in the instability strip and use their B) 100 million years variability period to measure their age. C) 1 billion years D. Look for the age of stars at the main-sequence turnoff point. D) 10 billion years E. Determine if the cluster is an open cluster or globular E) 100 billion years cluster and use the average age of those types of clusters. Clicker Question Where we see this best: Star Main sequence A-stars have masses about 3 times that of the Sun, and luminosities about 30 Clusters times that of the Sun. What is the age of a cluster • Groups of 100’s to which has a “turnoff” at A-stars? (Remember: millions of stars • All about the same The Sun’s lifetime ~ 10 billion years) distance (apparent A) 100 thousand years brightness tracks luminosity well) B) 100 million years • All formed about the same time (i.e. all are C) 1 billion years same age) D) 10 billion years • Range of different mass E) 100 billion years stars! 1.) Open Clusters • Pleiades: an “open cluster” of stars about 100 million years old • Loose groups of 1000’s of stars • Compare with Sun’s age of about 4.6 • This is where most BILLION years old stars in the Galaxy are born Cepheid Variable Stars 2.) Globular Clusters • Some stars vary in brightness because they • Generally much cannot achieve proper older- up to 13 balance between power welling up from the core BILLION years and power radiated from the surface • ~millions of stars, • Most pulsating variable stars inhabit an instability densely packed strip on the H-R diagram • The most luminous ones • Intense gravitational are known as Cepheid interactions variables: important for distance measurements A Which star is A Which star is most like our most like our Sun? Sun? D D B B B Luminosity Luminosity Luminosity Luminosity C C Clicker Clicker question question Temperature Temperature A Which of these A Which of these stars will have stars will have changed the least changed the least 10 billion years 10 billion years D from now? D from now? B B C Luminosity Luminosity Luminosity Luminosity C C Clicker Clicker question question Temperature Temperature A Which of these A Which of these stars can be no stars can be no more than 10 more than 10 million years old? million years old? D D A B B Luminosity Luminosity Luminosity Luminosity C Clicker C Clicker question question Temperature Temperature Stellar Properties Review Luminosity: from brightness and distance 10-4 L - 106 L (100 M ) (0.08 MSun) Sun Sun Sun Temperature: from color and spectral type (0.08 MSun) 3,000 K - 50,000 K (100 MSun) Mass: from period (p) and average separation (a) of binary-star orbit 0.08 MSun - 100 MSun .
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